This invention relates in general to electrophotographic imaging members and more specifically, to imaging members comprising charge generator layers and multiple charge transport layers containing charge transporting polymers.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is a good insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
An electrostatic latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of any suitable means well known in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the surface of the photoconductive not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image.
The photodischarge of the photoconductive layer requires that the layer photogenerate conductive charge and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking and an adhesive layer between the conductive layer and the photoconducting layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be optionally be employed.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Another type of charge transport layer has been developed which utilizes a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a group pendant from the backbone of the polymer backbone or as a moiety in the backbone of the polymer. These types of charge transport polymers include materials such as poly(N-vinylcarbazole), polysilylenes, and others including those described, for example, in U.S. Pat. No. 4,618,551, U.S. Pat. No. 4,806,443, U.S. Pat. No. 4,806,444, U.S. Pat. No. 4,818,650, U.S. Pat. No. 4,935,487, and U.S. Pat. No. 4,956,440. The disclosures of these patents are incorporated herein in their entirety.
Charge generator layers comprise amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like fabricated by vacuum evaporation or deposition. The charge generator layers may also comprise inorganic pigments of crystalline selenium and its alloys; Group II-VI compounds; and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos; and the like dispersed in a film forming polymeric binder and fabricated by solvent coating techniques.
Phthalocyanines have been employed as photogenerating materials for use in laser printers utilizing infrared exposure systems. Infrared sensitivity is required for photoreceptors exposed to low cost semiconductor laser diode light exposure devices. The absorption spectrum and photosensitivity of the phthalocyanines depend on the central metal atom of the compound. Many metal phthalocyanines have been reported and include, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine. The phthalocyanines exist in many crystal forms which have a strong influence on photogeneration.
One of the design criteria for the selection of the photosensitive pigment for a charge generator layer and the charge transporting molecule for a transport layer is that, when light photons photogenerate holes in the pigment, the holes be efficiently injected into the charge transporting molecule in the transport layer. More specifically, the injection efficiency from the pigment to the transport layer should be high. A second design criterion is that the injected holes be transported across the charge transport layer in a short time; shorter than the time duration between the exposure and development stations in an imaging device. The transit time across the transport layer is determined by the charge carrier mobility in the transport layer. The charge carrier mobility is the velocity per unit field and has dimensions of cm.sup.2 /volt sec. The charge carrier mobility is a function of the structure of the charge transporting molecule, the concentration of the charge transporting molecule in the transport layer and the electrically "inactive" binder polymer in which the charge transport molecule is dispersed. It is believed that the injection efficiency can be maximized by choosing a transport molecule whose ionization potential is lower than that of the pigment. However, low ionization potential molecules may have other deficiencies, one of which is their instability in an atmosphere of corona effluents. A copy quality defect resulting from the chemical interaction of the surface of the transport layer with corona effluents is referred to as "parking deletion" and is described in detail below.
Reprographic machines often utilize multilayered organic photoconductors and also employ corotrons or scorotrons to charge the photoconductors prior to imagewise exposure. During the operating lifetime of these photoconductors they are subjected to corona effluents which include ozone, various oxides of nitrogen etc. It is believed that some of these oxides of nitrogen are converted to nitric acid in the presence of water molecules present in the ambient operating atmosphere. The top surface of the photoconductor is exposed to the nitric acid during operation of the machine and photoconductor molecules at the very top surface of the transport layer are converted to what is believed to be the nitrated species of the molecules and these could form an electrically conductive film. However, during operation of the machine, the cleaning subsystem continuously removes (by wear) a region of the top surface thereby preventing accumulation of the conductive species. However, such is not the case when the machine is not operating (i.e. in idle mode) between two large copy runs. During the idle mode between long copy runs of, for example, 1000 copies, a specific segment of the photoreceptor comes to rest (is parked) beneath a corotron that had been in operation during the long copy run. Although the high voltage to the corotron is turned off during the time period when the photoreceptor is parked, some effluents (e.g. nitric acid, etc.) continue to be emitted from the corotron shield, corotron housing, etc. This effluent emission is concentrated in the region of the stationary photoreceptor parked directly underneath the corotron. The effluents render that surface region electrically conductive. When machine operation is resumed for the next copy run, a loss of resolution, and even deletion, is observed in the affected region. Thus, the corona induced changes primarily occur at the surface region of the charge transport layer. These changes are manifested in the form of increased conductivity which results in loss of resolution of the final toner images. In the case of severe increases in conductivity, there can be regions of severe deletions in the images. The problem of deletion sometimes manifests itself as a loss of resolution when the reprographic machine operates in an extended run of 1000 cycles or more. The onset of loss of resolution depends on the type and number of corotrons employed and the air-flow configuration within the machines.
One technique to obtain a highly sensitive and yet more corona stable photoreceptor is to have several transport layers with progressively increasing ionization potentials. The material with the lowest ionization potential is coated first, closest to the generator layer and transport layer with the highest ionization potential is coated last, near the free surface of the device. In this structure, the injection from the generator layer is still maintained at a high value because of the low ionization potential transport molecule closest to the generator layer. The hole injected into the first transport layer drifts through that layer and is injected into the second transport layer with slightly higher ionization potential and so on. By the proper selection of the materials, the difference in ionization potential between adjacent transport layers is held to a minimum, to prevent an injection barrier between transport layers. This scheme ought to result in a device with very high sensitivity and yet be stable in a corona environment. A representative device appears to be described in U.S. Pat. No. 4,889,784. If the transporting species are donor molecules, this scheme would require different classes of materials for the different transport layers. The ionization potentials of different classes of material such as diamine, stillbene and hydrazone are different and the ionization potentials can be changed further by the proper selection of substituent groups. However, the multi-transport layer scheme is not feasible with the small molecule transport layers because of intermixing between transport layers during the coating process resulting in migration of the molecule from the second transport layer into the first transport layer and so on with each additionally deposited layer.
It has also been found that photoreceptors containing an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder become unstable when employed with liquid development systems. These photoreceptors suffer from cracking, crazing, extraction, phase separation and crystallization of charge transporting active compounds by contact with the organic carrier fluid in a machine employing a liquid development system. A commonly employed organic carrier fluid in liquid development systems is an isoparaffinic hydrocarbon, for example, Isopar.RTM. available from Exxon Chemicals International, Inc. The leaching and crystallization of charge transporting active compounds markedly degrades the mechanical integrity and electro-optical performance of the photoreceptors. More specifically, the organic carrier fluid of a liquid developer leaches out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N'- diphenyl-N,N'-bis (3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine; bis-(4-diethylamino-2-methy lphenyl)-phenylmethane; 2,5-bis-(4'-diethylamino phenyl)-1,3,4-oxadiazole; 1-phenyl-3-(4'-diethylaminostyryl)-5-(4"-diethylaminophenyl)-pyrazoline; 1,1-bis-(4-(di-N,N'-p-methylphenyl)-aminophenyl)-cyclohexane; 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenylamino phenyl)-ethylene and the like. The leaching process results in crystallization of the charge transporting activating small molecules, such as the aforementioned arylamine compounds, onto the photoreceptor surface and subsequent migration of the arylamine into the liquid developer ink. In addition, the ink vehicle, typically a C.sub.10 -C.sub.14 branched hydrocarbon, induces the formation of cracks and crazes in the photoreceptor leading to the onset of copy defects and shortened photoreceptor life. Sufficient degradation can occur in less than eight hours of use making these photoreceptors unsuitable for use in machines employing liquid developers.